The cyclopropenium ion is the cation with the formula C
3H+
3. It has attracted attention as the smallest example of an aromatic cation. Its salts have been isolated, and many derivatives have been characterized by X-ray crystallography. [1] The cation and some simple derivatives have been identified in the atmosphere of the Saturnian moon Titan. [2]
With two π electrons, the cyclopropenium cation class obeys Hückel’s rules of aromaticity for 4n + 2 electrons since, in this case, n = 0. Consistent with this prediction, the C3H3 core is planar and the C–C bonds are equivalent. In the case of the cation in [C3(SiMe3)3]+SbCl−
6, [3] the ring C–C distances range from 1.374(2) to 1.392(2) Å.
![Structure of the salt [C3(SiMe3)3]SbCl
6 NEZKOS01.png](http://upload.wikimedia.org/wikipedia/commons/thumb/c/c6/NEZKOS01.png/220px-NEZKOS01.png)
Salts of many cyclopropenyl cations have been characterized. Their stability varies according to the steric and inductive effects of the substituents.
Salts of triphenylcyclopropenium were first reported by Ronald Breslow in 1957. The salt was prepared in two steps starting with the reaction of phenyldiazoacetonitrile with diphenylacetylene to yield 1,2,3-triphenyl-2-cyclopropene nitrile. Treatment of this with boron trifluoride yielded [C3Ph3]BF4. [4] [5] [6]
The parent cation, [C3H3]+, was reported as its hexachloroantimonate (SbCl−
6) salt in 1970. [7] It is indefinitely stable at −20 °C.
Trichlorocyclopropenium salts are generated by chloride abstraction from tetrachlorocyclopropene: [8]
Tetrachlorocyclopropene can be converted to tris(tert-butyldimethylsilyl)cyclopropene. Hydride abstraction with nitrosonium tetrafluoroborate yields the trisilyl-substituted cyclopropenium cation. [9]
Amino-substituted cyclopropenium salts are particularly stable. [10] [11] Calicene is an unusual derivative featuring cyclopropenium linked to a cyclopentadienide.
Chloride salts of cyclopropenium esters are intermediates in the use of dichlorocyclopropenes for the conversion of carboxylic acids to acid chlorides: [12]
Related cyclopropenium cations are produced in the regeneration of the 1,1-dichlorocyclopropenes from the cyclopropenones.
The cyclopropenium chlorides have been applied to peptide bond formation. [12] For example, in the figure below, reacting a boc-protected amino acid with an unprotected amino acid in the presence of the cyclopropenium ion allows the formation of a peptide bond via acid chloride formation followed by nucleophilic substitution with the unprotected amino acid.
This method of mildly generating acid chlorides can also be useful for linking alpha-anomeric sugars. [13] After using the cyclopropenium ion to form the chloride at the anomeric carbon, the compound is iodinated with tetrabutylammonium iodide. This iodine can thereafter be substituted by any ROH group to quickly undergo alpha-selective linkage of sugars.
Additionally, some synthetic routes make use of cyclopropenium ring openings yielding an allylcarbene cation. The linear degradation product yields both a nucleophilic and electrophilic carbon centers. [14]
Many complexes are known with cyclopropenium ligands. Examples include [M(C3Ph3)(PPh3)2]+ (M = Ni, Pd, Pt) and Co(C3Ph3)(CO)3. Such compounds are prepared by reaction of cyclopropenium salts with low valent metal complexes. [15]
Because many substituted derivatives are known, cyclopropenium salts have attracted attention as possible polyelectrolytes, relevant to technologies such as desalination and fuel cells. The tris(dialkylamino)cyclopropenium salts have been particularly evaluated because of their high stability. [16]
In chemistry, amines are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Formally, amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines, trimethylamine, and aniline. Inorganic derivatives of ammonia are also called amines, such as monochloramine.
The Friedel–Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877 to attach substituents to an aromatic ring. Friedel–Crafts reactions are of two main types: alkylation reactions and acylation reactions. Both proceed by electrophilic aromatic substitution.
The Sandmeyer reaction is a chemical reaction used to synthesize aryl halides from aryl diazonium salts using copper salts as reagents or catalysts. It is an example of a radical-nucleophilic aromatic substitution. The Sandmeyer reaction provides a method through which one can perform unique transformations on benzene, such as halogenation, cyanation, trifluoromethylation, and hydroxylation.
N-Bromosuccinimide or NBS is a chemical reagent used in radical substitution, electrophilic addition, and electrophilic substitution reactions in organic chemistry. NBS can be a convenient source of Br•, the bromine radical.
The Pictet–Spengler reaction is a chemical reaction in which a β-arylethylamine undergoes condensation with an aldehyde or ketone followed by ring closure. The reaction was first discovered in 1911 by Amé Pictet and Theodor Spengler. Traditionally, an acidic catalyst in protic solvent was employed with heating; however, the reaction has been shown to work in aprotic media in superior yields and sometimes without acid catalysis. The Pictet–Spengler reaction can be considered a special case of the Mannich reaction, which follows a similar reaction pathway. The driving force for this reaction is the electrophilicity of the iminium ion generated from the condensation of the aldehyde and amine under acid conditions. This explains the need for an acid catalyst in most cases, as the imine is not electrophilic enough for ring closure but the iminium ion is capable of undergoing the reaction.
Thiophenol is an organosulfur compound with the formula C6H5SH, sometimes abbreviated as PhSH. This foul-smelling colorless liquid is the simplest aromatic thiol. The chemical structures of thiophenol and its derivatives are analogous to phenols. An exception is the oxygen atom in the hydroxyl group (-OH) bonded to the aromatic ring is replaced by a sulfur atom. The prefix thio- implies a sulfur-containing compound and when used before a root word name for a compound which would normally contain an oxygen atom, in the case of 'thiol' that the alcohol oxygen atom is replaced by a sulfur atom.
A persistent carbene is an organic molecule whose natural resonance structure has a carbon atom with incomplete octet, but does not exhibit the tremendous instability typically associated with such moieties. The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (NHC), in which nitrogen atoms flank the formal carbene.
A carbenium ion is a positive ion with the structure RR′R″C+, that is, a chemical species with carbon atom having three covalent bonds, and it bears a +1 formal charge. But IUPAC confuses coordination number with valence, incorrectly considering carbon in carbenium as trivalent.
The Petasis reaction is the multi-component reaction of an amine, a carbonyl, and a vinyl- or aryl-boronic acid to form substituted amines.
Pyrylium is a cation with formula C5H5O+, consisting of a six-membered ring of five carbon atoms, each with one hydrogen atom, and one positively charged oxygen atom. The bonds in the ring are conjugated as in benzene, giving it an aromatic character. In particular, because of the positive charge, the oxygen atom is trivalent. Pyrilium is a mono-cyclic and heterocyclic compound, one of the oxonium ions.
The Stetter reaction is a reaction used in organic chemistry to form carbon-carbon bonds through a 1,4-addition reaction utilizing a nucleophilic catalyst. While the related 1,2-addition reaction, the benzoin condensation, was known since the 1830s, the Stetter reaction was not reported until 1973 by Dr. Hermann Stetter. The reaction provides synthetically useful 1,4-dicarbonyl compounds and related derivatives from aldehydes and Michael acceptors. Unlike 1,3-dicarbonyls, which are easily accessed through the Claisen condensation, or 1,5-dicarbonyls, which are commonly made using a Michael reaction, 1,4-dicarbonyls are challenging substrates to synthesize, yet are valuable starting materials for several organic transformations, including the Paal–Knorr synthesis of furans and pyrroles. Traditionally utilized catalysts for the Stetter reaction are thiazolium salts and cyanide anion, but more recent work toward the asymmetric Stetter reaction has found triazolium salts to be effective. The Stetter reaction is an example of umpolung chemistry, as the inherent polarity of the aldehyde is reversed by the addition of the catalyst to the aldehyde, rendering the carbon center nucleophilic rather than electrophilic.
The Scholl reaction is a coupling reaction between two arene compounds with the aid of a Lewis acid and a protic acid. It is named after its discoverer, Roland Scholl, a Swiss chemist.
The Stieglitz rearrangement is a rearrangement reaction in organic chemistry which is named after the American chemist Julius Stieglitz (1867–1937) and was first investigated by him and Paul Nicholas Leech in 1913. It describes the 1,2-rearrangement of trityl amine derivatives to triaryl imines. It is comparable to a Beckmann rearrangement which also involves a substitution at a nitrogen atom through a carbon to nitrogen shift. As an example, triaryl hydroxylamines can undergo a Stieglitz rearrangement by dehydration and the shift of a phenyl group after activation with phosphorus pentachloride to yield the respective triaryl imine, a Schiff base.
Oxazoline is a five-membered heterocyclic organic compound with the formula C3H5NO. It is the parent of a family of compounds called oxazolines, which contain non-hydrogenic substituents on carbon and/or nitrogen. Oxazolines are the unsaturated analogues of oxazolidines, and they are isomeric with isoxazolines, where the N and O are directly bonded. Two isomers of oxazoline are known, depending on the location of the double bond.
In organic chemistry, carbonyl reduction is the conversion of any carbonyl group, usually to an alcohol. It is a common transformation that is practiced in many ways. Ketones, aldehydes, carboxylic acids, esters, amides, and acid halides - some of the most pervasive functional groups, -comprise carbonyl compounds. Carboxylic acids, esters, and acid halides can be reduced to either aldehydes or a step further to primary alcohols, depending on the strength of the reducing agent. Aldehydes and ketones can be reduced respectively to primary and secondary alcohols. In deoxygenation, the alcohol group can be further reduced and removed altogether by replacement with H.
Rhodocene is a chemical compound with the formula [Rh(C5H5)2]. Each molecule contains an atom of rhodium bound between two planar aromatic systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compound as it has (haptic) covalent rhodium–carbon bonds. The [Rh(C5H5)2] radical is found above 150 °C (302 °F) or when trapped by cooling to liquid nitrogen temperatures (−196 °C [−321 °F]). At room temperature, pairs of these radicals join via their cyclopentadienyl rings to form a dimer, a yellow solid.
The Minisci reaction is a named reaction in organic chemistry. It is a nucleophilic radical substitution to an electron deficient aromatic compound, most commonly the introduction of an alkyl group to a nitrogen containing heterocycle. The reaction was published in 1971 by F. Minisci. In the case of N-Heterocycles, the conditions must be acidic to ensure protonation of said heterocycle. A typical reaction is that between pyridine and pivalic acid with silver nitrate, sulfuric acid and ammonium persulfate to form 2-tert-butylpyridine. The reaction resembles Friedel-Crafts alkylation but with opposite reactivity and selectivity.
Tetramethylurea is the organic compound with the formula (Me2N)2CO. It is a substituted urea. This colorless liquid is used as an aprotic-polar solvent, especially for aromatic compounds and is used e. g. for Grignard reagents.
Hydroxylamine-O-sulfonic acid (HOSA) or aminosulfuric acid is the inorganic compound with molecular formula H3NO4S that is formed by the sulfonation of hydroxylamine with oleum. It is a white, water-soluble and hygroscopic, solid, commonly represented by the condensed structural formula H2NOSO3H, though it actually exists as a zwitterion and thus is more accurately represented as +H3NOSO3−. It is used as a reagent for the introduction of amine groups (–NH2), for the conversion of aldehydes into nitriles and alicyclic ketones into lactams (cyclic amides), and for the synthesis of variety of nitrogen-containing heterocycles.
Tris(dimethylamino)methane (TDAM) is the simplest representative of the tris(dialkylamino)methanes of the general formula (R2N)3CH in which three of the four of methane's hydrogen atoms are replaced by dimethylamino groups (−N(CH3)2). Tris(dimethylamino)methane can be regarded as both an amine and an orthoamide.